Active Pixel Sensors (APS) are solid state imagers wherein each pixel contains a photo-sensing means and some other active devices that perform control functions on the pixel. Passive pixel sensors (PPS) are imagers having photosensing means and address transistor, but no active components. Recent and prior art devices have focused on using commercially available CMOS foundry processes to manufacture APS and PPS devices. The use of CMOS to manufacture APS and PPS devices has a resulting advantage of easily integrating signal processing and control circuits on the same chip as the imager. Thus, making it easier to fabricate a camera on a single semiconductor device, and providing a low cost integrated digital imaging device.
In APS and PPS devices typically fabricated using standard CMOS processes, the photodetector within the pixel has been either a photocapacitor, (also referred to as a photogate), or a photodiode. Photogate detectors have poor blue quantum efficiency due to the absorption of short wavelength light in the gate material, typically polysilicon, that covers the photo-sensing area. Additionally, photogate detectors require a double level polysilicon process to provide reasonable fill factor, (fill factor being defined as the percentage of the entire pixel area that is the photodetector). Double poly processes are not typically available, and are more complex and costly when compared to single level polysilicon processes. Photodiode detectors have high dark current, reduced blue quantum efficiency, and image lag. High dark current is attributable to the use of heavily doped n-type regions, that are typically used as NMOS sources and drains, as the photodiode. In those devices with heavily doped implants, the implant damage is not easily annealed since the goal of CMOS processes is to achieve very shallow sources and drains, having low resistivity. Therefore, the transistor gate length can be minimized and transistor speed maximized. It is not critical for CMOS sources and drains to have low dark current. Additionally, the silicon-silicon/dioxide interface states can contribute to dark current and recombination of shallow photo-electrons in the photodiode, further increasing dark current and degrading blue quantum efficiency.
Image lag is a phenomenon that exists within many conventional CMOS imagers that can result in ghost image artifacts. Image lag results from the inability to completely reset a photodiode in the short amount of time due to the large capacitance associated within the photodiode and reset by sub-threshold current. This causes photoelectrons to be left within the photodiode and inadvertently be read as signal electrons corresponding to the next frame in the image sequence, causing ghost images. These deficiencies described above lead to image quality that is not suitable for many digital imaging applications.
A pinned photodiode APS and PPS was disclosed by P. Lee et al. in allowed U.S. patent application, Ser. No. 08/421,173, (now issued as U.S. Pat. No. 5,625,210). This disclosure illustrates an active pixel sensor that overcomes the limitations of the photodetectors previously discussed. However the pinned photodiode APS and PPS device has a smaller fill factor than a photodiode based APS and PPS device, which results in lower overall sensitivity.
From the foregoing discussion it is apparent that there remains a need within the art to provide an APS and PPS pixel structure that provides low dark current, high blue quantum efficiency, low image lag, and high fill factor.